ACT® Science

Cellular Molecules

ACTSCI-0JU8RK

Use the information below, as well as the original passage, to answer this question.

Scientists have always had a great curiosity about cancer cells. When grown in culture dishes, cancer cells will divide indefinitely, unlike normal actively dividing cells such as fibroblasts, which eventually give out and enter apoptosis. As new research about telomeres became known, oncologists decided to re-examine cells from various abnormal growths. What they saw was stunning, the telomeres of these cells never shrank, nor did the ending DNA fibers ever fray. Whether it was the first division or the 500th, these cell telomeres retained their original base pair number and their ability to attract the protein caps necessary to keep their DNA strands from fraying.

The question that was quickly asked is what substance is responsible for keeping these abnormal cells telomere’s healthy? Four biological molecules known to exist in actively dividing cells were identified as possible candidates. A study of the concentrations of these molecules were undertaken to see if one could possibly be the telomeres “magic bullet”.

The data table below shows the results of a study using five different types of cells from human donors grown in the same culture medium. Chemical assays were used to measure the concentration of each molecule in each cell type.

$$ \text{Concentration in ug/ml} $$

Cell Type

Rate of Cell Division

DNA Polymerase

Housekeeping Protein

Telomerase

Spindle Microbtube Protein

Liver

Medium

1.4

2.5

0.4

0.6

Neuron

Very low

0.2

2.5

0.1

0.1

Fibrolast

High

1.7

2.5

1.2

0.9

Bone Sarcoma

Very High

2.1

2.5

6.1

0.9

Breast Carcinoma

Very High

2.2

2.5

6.1

0.9

Telomeres are DNA sequences that repeat at the end of chromosomes. They protect chromosomes by attracting protein caps. These caps bind to the chromosomes and keep the long, thin threads of DNA from fraying. Usually the sequences are about 200 base pairs (bp) long and repeat 400-500 times at the end of each chromosome. When a cell is going to divide it enters a part of its life cycle called the synthesis phase. During this time a protein molecule called DNA polymerase will make a copy of each chromosome so that each new cell after division will have its own set of nucleotide bases. However DNA polymerase can never quite make it to the very end of each chromosome, so that after each cell division, the telomeres have shortened.

Eventually, the telomere sequences are too short to attract the protein caps, and without the proper protection, the delicate DNA threads fray, and the cell can no longer make the proteins it needs to divide, the cell enters a phase known as apoptosis, or programmed cell death.

Telomeres have attracted a great deal of research and much has been discovered their role in cell division. One of the first experiments was designed to elucidate exactly how much telomere DNA was needed in order for a cell to be healthy enough to divide. The researcher involved used cultures of human skin cells, called fibroblasts, to test the hypothesis. Skin cells were used because they are known to grow and divide actively for a number of generations. Cells were cultured in normal growth media (a cell sample is put in a petri dish and fed, the cells then divide on their own) and the number of cell divisions until apoptosis were measured. The length and condition of the telomeres were measured and recorded after the 10th division and then after apoptosis for each culture. The data is shown below:

Culture Plate #

# of Cell Divisions until Apoptosis

Telomere length after 10th division (bp)

Telomere length after 20th division (bp)

Telomere length just before apoptosis

1

23

8000

6000

3300

2

22

7500

5500

3450

3

24

8400

6400

3200

4

23

8000

6000

3300

5

26

9000

7000

3200

Based on the results in the data table, which combination of cellular molecules do actively dividing cells need?